Power System Efficiency - Transformer
Power System Efficiency - Transformer
Power System Efficiency - Transformer
ELECTRICAL SYSTEM
1 Bureau of Energy Efficiency
Syllabus
Electrical system: Electricity billing, Electrical load management and maximum demand
control, Power factor improvement and its benefit, Selection and location of capacitors,
Performance assessment of PF capacitors, Distribution and transformer losses.
1.1 Introduction to Electric Power Supply Systems
Electric power supply system in a country comprises of generating units that produce electric-
ity; high voltage transmission lines that transport electricity over long distances; distribution
lines that deliver the electricity to consumers; substations that connect the pieces to each other;
and energy control centers to coordinate the operation of the components.
The Figure 1.1 shows a simple electric supply system with transmission and distribution
network and linkages from electricity sources to end-user.
Figure 1.1 Typical Electric Power Supply Systems
Power Generation Plant
The fossil fuels such as coal, oil and natural gas, nuclear energy, and falling water (hydel) are
commonly used energy sources in the power generating plant. A wide and growing variety of
unconventional generation technologies and fuels have also been developed, including cogen-
eration, solar energy, wind generators, and waste materials.
About 70 % of power generating capacity in India is from coal based thermal power plants.
The principle of coal-fired power generation plant is shown in Figure 1.2. Energy stored in the
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coal is converted in to electricity in thermal power plant. Coal is pulverized to the consistency
of talcum powder. Then powdered coal is blown into the water wall boiler where it is burned at
temperature higher than 1300C. The heat in the combustion gas is transferred into steam. This
high-pressure steam is used to run the steam turbine to spin. Finally turbine rotates the genera-
tor to produce electricity.
Figure 1.2 Principle of Thermal Power Generation
In India, for the coal based power plants, the overall efficiency ranges from 28% to 35%
depending upon the size, operational practices and capacity utilization. Where fuels are the
source of generation, a common term used is the HEAT RATE which reflects the efficiency
of generation. HEAT RATE is the heat input in kilo Calories or kilo Joules, for generating
one kilo Watt-hour of electrical output. One kilo Watt hour of electrical energy being equiv-
alent to 860 kilo Calories of thermal energy or 3600 kilo Joules of thermal energy. The HEAT
RATE expresses in inverse the efficiency of power generation.
Transmission and Distribution Lines
The power plants typically produce 50 cycle/second
(Hertz), alternating-current (AC) electricity with volt-
ages between 11kV and 33kV. At the power plant site,
the 3-phase voltage is stepped up to a higher voltage for
transmission on cables strung on cross-country towers.
High voltage (HV) and extra high voltage (EHV)
transmission is the next stage from power plant to
transport A.C. power over long distances at voltages
like; 220 kV & 400 kV. Where transmission is over
1000 kM, high voltage direct current transmission is
also favoured to minimize the losses.
Sub-transmission network at 132 kV, 110 kV, 66 kV
or 33 kV constitutes the next link towards the end user.
Distribution at 11 kV / 6.6 kV / 3.3 kV constitutes the
last link to the consumer, who is connected directly or
through transformers depending upon the drawl level of
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service. The transmission and distribution network include sub-stations, lines and distribution
transformers. High voltage transmission is used so that smaller, more economical wire sizes can
be employed to carry the lower current and to reduce losses. Sub-stations, containing step-down
transformers, reduce the voltage for distribution to industrial users. The voltage is further
reduced for commercial facilities. Electricity must be generated, as and when it is needed since
electricity cannot be stored virtually in the system.
There is no difference between a transmission line and a distribution line except for the volt-
age level and power handling capability. Transmission lines are usually capable of transmitting
large quantities of electric energy over great distances. They operate at high voltages.
Distribution lines carry limited quantities of power over shorter distances.
Voltage drops in line are in relation to the resistance and reactance of line, length and the
current drawn. For the same quantity of power handled, lower the voltage, higher the current
drawn and higher the voltage drop. The current drawn is inversely proportional to the voltage
level for the same quantity of power handled.
The power loss in line is proportional to resistance and square of current. (i.e. P
LOSS
=I
2
R).
Higher voltage transmission and distribution thus would help to minimize line voltage drop in
the ratio of voltages, and the line power loss in the ratio of square of voltages. For instance, if
distribution of power is raised from 11 kV to 33 kV, the voltage drop would be lower by a fac-
tor 1/3 and the line loss would be lower by a factor (1/3)
2
i.e., 1/9. Lower voltage transmission
and distribution also calls for bigger size conductor on account of current handling capacity
needed.
Cascade Efficiency
The primary function of transmission and distribution equipment is to transfer power econom-
ically and reliably from one location to another.
Conductors in the form of wires and cables strung on towers and poles carry the high-volt-
age, AC electric current. A large number of copper or aluminum conductors are used to form
the transmission path. The resistance of the long-distance transmission conductors is to be min-
imized. Energy loss in transmission lines is wasted in the form of I
2
R losses.
Capacitors are used to correct power factor by causing the current to lead the voltage. When
the AC currents are kept in phase with the voltage, operating efficiency of the system is main-
tained at a high level.
Circuit-interrupting devices are switches, relays, circuit breakers, and fuses. Each of these
devices is designed to carry and interrupt certain levels of current. Making and breaking the cur-
rent carrying conductors in the transmission path with a minimum of arcing is one of the most
important characteristics of this device. Relays sense abnormal voltages, currents, and frequen-
cy and operate to protect the system.
Transformers are placed at strategic locations throughout the system to minimize power
losses in the T&D system. They are used to change the voltage level from low-to-high in step-
up transformers and from high-to-low in step-down units.
The power source to end user energy efficiency link is a key factor, which influences the
energy input at the source of supply. If we consider the electricity flow from generation to the
user in terms of cascade energy efficiency, typical cascade efficiency profile from generation to
11 33 kV user industry will be as below:
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Generation
Efficiency
1
Step-up Station
2
EHV
Transmission &
Station
3
HV
Transmission &
Station
4
Sub-transmission
5
Primary
Distribution
7
Distribution
Station
6
End user
Premises
The cascade efficiency in the T&D system from output of the power plant to the end use is
87% (i.e. 0.995 x 0.99 x 0.975 x 0.96 x 0.995 x 0.95 = 87%)
Industrial End User
At the industrial end user premises, again the plant network elements like transformers at
receiving sub-station, switchgear, lines and cables, load-break switches, capacitors cause loss-
es, which affect the input-received energy. However the losses in such systems are meager and
unavoidable.
A typical plant single line diagram of electrical distribution system is shown in Figure 1.3
Efficiency ranges 28 35 % with respect to size of thermal plant,
age of plant and capacity utilisation
Step-up to 400 / 800 kV to enable EHV transmission
Envisaged max. losses 0.5 % or efficiency of 99.5 %
EHV transmission and substations at 400 kV / 800 kV.
Envisaged maximum losses 1.0 % or efficiency of 99 %
HV transmission & Substations for 220 / 400 kV.
Envisaged maximum losses 2.5 % or efficiency of 97.5 %
Sub-transmission at 66 / 132 kV
Envisaged maximum losses 4 % or efficiency of 96 %
Step-down to a level of 11 / 33 kV.
Envisaged losses 0.5 % or efficiency of 99.5 %
Distribution is final link to end user at 11 / 33 kV.
Envisaged losses maximum 5 % of efficiency of 95 %
Cascade efficiency from Generation to end user
=
1
x
2
x
3
x
4
x
5
x
6
x
7
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ONE Unit saved = TWO Units Generated
After power generation at the plant it is transmitted and distributed over a wide network.
The standard technical losses are around 17 % in India (Efficiency = 83%). But the figures for
many of the states show T & D losses ranging from 17 50 %. All these may not constitute
technical losses, since un-metered and pilferage are also accounted in this loss.
When the power reaches the industry, it meets the transformer. The energy efficiency of the
transformer is generally very high. Next, it goes to the motor through internal plant distribution
network. Atypical distribution network efficiency including transformer is 95% and motor effi-
ciency is about 90%. Another 30 % (Efficiency =70%)is lost in the mechanical system which
includes coupling/ drive train, a driven equipment such as pump and flow control valves/throt-
tling etc. Thus the overall energy efficiency becomes 50%. (0.83 x 0.95x 0.9 x 0.70 = 0.50, i.e.
50% efficiency)
Hence one unit saved in the end user is equivalent to two units generated in the power plant.
(1Unit / 0.5Eff = 2 Units)
1.2 Electricity Billing
The electricity billing by utilities for medium & large enterprises, in High Tension (HT) cate-
gory, is often done on two-part tariff structure, i.e. one part for capacity (or demand) drawn and
the second part for actual energy drawn during the billing cycle. Capacity or demand is in kVA
(apparent power) or kW terms. The reactive energy (i.e.) kVArh drawn by the service is also
TRIVECTOR METER
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recorded and billed for in some utilities, because this would affect the load on the utility.
Accordingly, utility charges for maximum demand, active energy and reactive power drawn (as
reflected by the power factor) in its billing structure. In addition, other fixed and variable
expenses are also levied.
The tariff structure generally includes the following components:
a) Maximum demand Charges
These charges relate to maximum demand registered during month/billing period and
corresponding rate of utility.
b) Energy Charges
These charges relate to energy (kilowatt hours) consumed during month / billing
period and corresponding rates, often levied in slabs of use rates. Some utilities now
charge on the basis of apparent energy (kVAh), which is a vector sum of kWh and
kVArh.
c) Power factor penalty or bonus rates, as levied by most utilities, are to contain reactive
power drawn from grid.
d) Fuel cost adjustment charges as levied by some utilities are to adjust the increasing fuel
expenses over a base reference value.
e) Electricity duty charges levied w.r.t units consumed.
f) Meter rentals
g) Lighting and fan power consumption is often at higher rates, levied sometimes on slab
basis or on actual metering basis.
h) Time Of Day (TOD) rates like peak and non-peak hours are also prevalent in tariff
structure provisions of some utilities.
i) Penalty for exceeding contract demand
j) Surcharge if metering is at LT side in some of the utilities
Analysis of utility bill data and monitoring its trends helps energy manager to identify ways
for electricity bill reduction through available provisions in tariff framework, apart from ener-
gy budgeting.
The utility employs an electromagnetic or electronic trivector meter, for billing purposes.
The minimum outputs from the electromagnetic meters are
Maximum demand registered during the month, which is measured in preset time inter-
vals (say of 30 minute duration) and this is reset at the end of every billing cycle.
1
= Existing (Cos
-1
PF
1
) and
2
= Improved (Cos
-1
PF
2
)
Figure 1.8 Power factor before and after Improvement
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Alternatively the Table 1.2 can be used for capacitor sizing.
The figures given in table are the multiplication factors which are to be multiplied with the input
power (kW) to give the kVAr of capacitance required to improve present power factor to a new
desired power factor.
Example:
The utility bill shows an average power factor of 0.72 with an average KWof 627. How much
kVAr is required to improve the power factor to .95 ?
Using formula
Cos 1 = 0.72 , tan 1 = 0.963
Cos 2 = 0.95 , tan 2 = 0.329
kVAr required = P ( tan
1
- tan
2
) = 627 (0.964 0.329)
= 398 kVAr
Using table (see Table 1.2)
1) Locate 0.72 (original power factor) in column (1).
2) Read across desired power factor to 0.95 column. We find 0.635 multiplier
3) Multiply 627 (average kW) by 0.635 = 398 kVAr.
4) Install 400 kVAr to improve power factor to 95%.
Location of Capacitors
The primary purpose of capacitors is to reduce the maximum demand. Additional benefits are
derived by capacitor location. The Figure 1.9 indicates typical capacitor locations. Maximum
benefit of capacitors is derived by locating them as close as possible to the load. At this loca-
tion, its kVAr are confined to the smallest possible segment, decreasing the load current. This,
in turn, will reduce power losses of the
system substantially. Power losses are
proportional to the square of the cur-
rent. When power losses are reduced,
voltage at the motor increases; thus,
motor performance also increases.
Locations C1A, C1B and C1C of
Figure 1.9 indicate three different
arrangements at the load. Note that in
all three locations extra switches are
not required, since the capacitor is
either switched with the motor starter
or the breaker before the starter. Case
C1A is recommended for new installa-
tion, since the maximum benefit is
derived and the size of the motor ther-
mal protector is reduced. In Case C1B,
as in Case C1A, the capacitor is ener-
gized only when the motor is in opera-
Figure 1.9: Power Distribution Diagram Illustrating
Capacitor Locations
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TABLE 1.2 MULTIPLIERS TO DETERMINE CAPACITOR kVAr REQUIREMENTS FOR
POWER FACTOR CORRECTION
tion. Case C1B is recommended in cases where the installation already exists and the thermal
protector does not need to be re-sized. In position C1C, the capacitor is permanently connected
to the circuit but does not require a separate switch, since capacitor can be disconnected by the
breaker before the starter.
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It should be noted that the rating of the capacitor should not be greater than the no-load
magnetizing kVAr of the motor. If this condition exists, damaging over voltage or transient
torques can occur. This is why most motor manufacturers specify maximum capacitor ratings
to be applied to specific motors.
The next preference for capacitor locations as illustrated by Figure 1.9 is at locations C2 and
C3. In these locations, a breaker or switch will be required. Location C4 requires a high volt-
age breaker. The advantage of locating capacitors at power centres or feeders is that they can
be grouped together. When several motors are running intermittently, the capacitors are per-
mitted to be on line all the time, reducing the total power regardless of load.
From energy efficiency point of view, capacitor location at receiving substation only helps
the utility in loss reduction. Locating capacitors at tail end will help to reduce loss reduction
within the plants distribution network as well and directly benefit the user by reduced
consumption. Reduction in the distribution loss % in kWh when tail end power factor is raised
from PF1 to a new power factor PF2, will be proportional to
Capacitors for Other Loads
The other types of load requiring capacitor application include induction furnaces, induction
heaters and arc welding transformers etc. The capacitors are normally supplied with control
gear for the application of induction furnaces and induction heating furnaces. The PF of arc fur-
naces experiences a wide variation over melting cycle as it changes from 0.7 at starting to 0.9
at the end of the cycle. Power factor for welding transformers is corrected by connecting capac-
itors across the primary winding of the transformers, as the normal PF would be in the range of
0.35.
Performance Assessment of Power Factor Capacitors
Voltage effects: Ideally capacitor voltage rating is to match the supply voltage. If the supply
voltage is lower, the reactive kVAr produced will be the ratio V
1
2
/V
2
2
where V
1
is the actual
supply voltage, V
2
is the rated voltage.
On the other hand, if the supply voltage exceeds rated voltage, the life of the capacitor is
adversely affected.
Material of capacitors: Power factor capacitors are available in various types by dielectric
material used as; paper/ polypropylene etc. The watt loss per kVAr as well as life vary with
respect to the choice of the dielectric material and hence is a factor to be considered while selec-
tion.
Connections: Shunt capacitor connections are adopted for almost all industry/ end user appli-
cations, while series capacitors are adopted for voltage boosting in distribution networks.
Operational performance of capacitors: This can be made by monitoring capacitor charging
current vis- a- vis the rated charging current. Capacity of fused elements can be replenished as
per requirements. Portable analyzers can be used for measuring kVAr delivered as well as
charging current. Capacitors consume 0.2 to 6.0 Watt per kVAr, which is negligible in compar-
ison to benefits.
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Some checks that need to be adopted in use of capacitors are :
i) Nameplates can be misleading with respect to ratings. It is good to check by charging
currents.
ii) Capacitor boxes may contain only insulated compound and insulated terminals with no
capacitor elements inside.
iii) Capacitors for single phase motor starting and those used for lighting circuits for volt-
age boost, are not power factor capacitor units and these cannot withstand power sys-
tem conditions.
1.5 Transformers
A transformer can accept energy at one voltage and deliver
it at another voltage. This permits electrical energy to be
generated at relatively low voltages and transmitted at high
voltages and low currents, thus reducing line losses and
voltage drop (see Figure 1.10).
Transformers consist of two or more coils that are elec-
trically insulated, but magnetically linked. The primary coil
is connected to the power source and the secondary coil
connects to the load. The turns ratio is the ratio between the
number of turns on the secondary to the turns on the prima-
ry (See Figure 1.11).
The secondary voltage is equal to the primary voltage
times the turns ratio. Ampere-turns are calculated by multi-
plying the current in the coil times the number of turns. Primary ampere-turns are equal to sec-
ondary ampere-turns. Voltage regulation of a transformer is the percent increase in voltage from
full load to no load.
Types of Transformers
Transformers are classified as two categories: power transformers
and distribution transformers.
Power transformers are used in transmission network of higher
voltages, deployed for step-up and step down transformer applica-
tion (400 kV, 200 kV, 110 kV, 66 kV, 33kV)
Distribution transformers are used for lower voltage distribu-
tion networks as a means to end user connectivity. (11kV, 6.6 kV,
3.3 kV, 440V, 230V)
Rating of Transformer
Rating of the transformer is calculated based on the connected load
and applying the diversity factor on the connected load, applicable
to the particular industry and arrive at the kVA rating of the
Transformer. Diversity factor is defined as the ratio of overall max-
imum demand of the plant to the sum of individual maximum demand of various equipment.
Diversity factor varies from industry to industry and depends on various factors such as
Figure 1.10 View of a Transformer
Figure 1.11
Transformer Coil
individual loads, load factor and future expansion needs of the plant. Diversity factor will
always be less than one.
Location of Transformer
Location of the transformer is very important as far as distribution loss is concerned.
Transformer receives HT voltage from the grid and steps it down to the required voltage.
Transformers should be placed close to the load centre, considering other features like optimi-
sation needs for centralised control, operational flexibility etc. This will bring down the distri-
bution loss in cables.
Transformer Losses and Efficiency
The efficiency varies anywhere between 96 to 99 percent. The efficiency of the transformers
not only depends on the design, but also, on the effective operating load.
Transformer losses consist of two parts: No-load loss and Load loss
1. No-load loss (also called core loss) is the power consumed to sustain the magnetic field
in the transformer's steel core. Core loss occurs whenever the transformer is energized;
core loss does not vary with load. Core losses are caused by two factors: hysteresis and
eddy current losses. Hysteresis loss is that energy lost by reversing the magnetic field in
the core as the magnetizing AC rises and falls and reverses direction. Eddy current loss
is a result of induced currents circulating in the core.
2. Load loss (also called copper loss) is associated with full-load current flow in the trans-
former windings. Copper loss is power lost in the primary and secondary windings of a
transformer due to the ohmic resistance of the windings. Copper loss varies with the
square of the load current. (P = I
2
R).
Transformer losses as a percentage of load is given in the Figure 1.12.
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Figure 1.12 Transformer loss vs %Load
For a given transformer, the manufacturer can supply values for no-load loss, P
NO-LOAD
, and
load loss, P
LOAD
. The total transformer loss, P
TOTAL,
at any load level can then be calculated
from:
P
TOTAL
= P
NO-LOAD
+ (% Load/100)
2
x P
LOAD
Where transformer loading is known, the actual transformers loss at given load can be com-
puted as:
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Voltage Fluctuation Control
A control of voltage in a transformer is important due to frequent changes in supply voltage
level. Whenever the supply voltage is less than the optimal value, there is a chance of nuisance
tripping of voltage sensitive devices. The voltage regulation in transformers is done by altering
the voltage transformation ratio with the help of tapping.
There are two methods of tap changing facility available: Off-circuit tap changer and
On-load tap changer.
Off-circuit tap changer
It is a device fitted in the transformer, which is used to vary the voltage transformation ratio.
Here the voltage levels can be varied only after isolating the primary voltage of the transformer.
On load tap changer (OLTC)
The voltage levels can be varied without isolating the connected load to the transformer. To
minimise the magnetisation losses and to reduce the nuisance tripping of the plant, the main
transformer (the transformer that receives supply from the grid) should be provided with On
Load Tap Changing facility at design stage. The down stream distribution transformers can be
provided with off-circuit tap changer.
The On-load gear can be put in auto mode or manually depending on the requirement.
OLTC can be arranged for transformers of size 250 kVA onwards. However, the necessity of
OLTC below 1000 kVA can be considered after calculating the cost economics.
Parallel Operation of Transformers
The design of Power Control Centre (PCC) and Motor Control Centre (MCC) of any new plant
should have the provision of operating two or more transformers in parallel. Additional
switchgears and bus couplers should be provided at design stage.
Whenever two transformers are operating in parallel, both should be technically identical in
all aspects and more importantly should have the same impedance level. This will minimise the
circulating current between transformers.
Where the load is fluctuating in nature, it is preferable to have more than one transformer
running in parallel, so that the load can be optimised by sharing the load between
transformers. The transformers can be operated close to the maximum efficiency range by
this operation.
1.6 System Distribution Losses
In an electrical system often the constant no load losses and the variable load losses are to be
assessed alongside, over long reference duration, towards energy loss estimation.
Identifying and calculating the sum of the individual contributing loss components is a chal-
lenging one, requiring extensive experience and knowledge of all the factors impacting the
operating efficiencies of each of these components.
For example the cable losses in any industrial plant will be up to 6 percent depending on the
size and complexity of the distribution system. Note that all of these are current dependent, and
can be readily mitigated by any technique that reduces facility current load. Various losses in
distribution equipment is given in the Table1.3.
In system distribution loss optimization, the various options available include:
Relocating transformers and sub-stations near to load centers
Re-routing and re-conductoring such feeders and lines where the losses / voltage drops
are higher.
Power factor improvement by incorporating capacitors at load end.
Optimum loading of transformers in the system.
Opting for lower resistance All Aluminum Alloy Conductors (AAAC) in place of
conventional Aluminum Cored Steel Reinforced (ACSR) lines
Minimizing losses due to weak links in distribution network such as jumpers, loose
contacts, old brittle conductors.
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TABLE 1.3 LOSSES IN ELECTRICAL DISTRIBUTION EQUIPMENT
S.No Equipment % Energy Loss at Full
Load Variations
Min Max
1. Outdoor circuit breaker (15 to 230 KV) 0.002 0.015
2. Generators 0.019 3.5
3. Medium voltage switchgears (5 to 15 KV) 0.005 0.02
4. Current limiting reactors 0.09 0.30
5. Transformers 0.40 1.90
6. Load break switches 0.003 0.0 25
7. Medium voltage starters 0.02 0.15
8. Bus ways less than 430 V 0.05 0.50
9. Low voltage switchgear 0.13 0.34
10. Motor control centers 0.01 0.40
11. Cables 1.00 4.00
12. Large rectifiers 3.0 9.0
13. Static variable speed drives 6.0 15.0
14. Capacitors (Watts / kVAr) 0.50 6.0
1.7 Harmonics
In any alternating current network, flow of current depends upon the voltage applied and the
impedance (resistance to AC) provided by elements like resistances, reactances of inductive and
capacitive nature. As the value of impedance in above devices is constant, they are called lin-
ear whereby the voltage and current relation is of linear nature.
However in real life situation, various devices like diodes, silicon controlled rectifiers,
PWM systems, thyristors, voltage & current chopping saturated core reactors, induction & arc
furnaces are also deployed for various requirements and due to their varying impedance char-
acteristic, these NON LINEAR devices cause distortion in voltage and current waveforms
which is of increasing concern in recent times. Harmonics occurs as spikes at intervals which
are multiples of the mains (supply) frequency and these distort the pure sine wave form of the
supply voltage & current.
Harmonics are multiples of the fundamental frequency of an electrical power system. If, for
example, the fundamental frequency is 50 Hz, then the 5th harmonic is five times that frequen-
cy, or 250 Hz. Likewise, the 7th harmonic is seven times the fundamental or 350 Hz, and so on
for higher order harmonics.
Harmonics can be discussed in terms of current or voltage. A5th harmonic current is simply
a current flowing at 250 Hz on a 50 Hz system. The 5th harmonic current flowing through the
system impedance creates a 5th harmonic voltage. Total Harmonic Distortion (THD) expresses
the amount of harmonics. The following is the formula for calculating the THD for current:
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When harmonic currents flow in a power system, they are known as poor power quality
or dirty power. Other causes of poor power quality include transients such as voltage spikes,
surges, sags, and ringing. Because they repeat every cycle, harmonics are regarded as a steady-
state cause of poor power quality.
When expressed as a percentage of fundamental voltage THD is given by,
THD
voltage
=
where V
1
is the fundamental frequency voltage and V
n
is n
th
harmonic voltage component.
Major Causes Of Harmonics
Devices that draw non-sinusoidal currents when a sinusoidal voltage is applied create harmon-
ics. Frequently these are devices that convert AC to DC. Some of these devices are listed below:
Electronic Switching Power Converters